The Optokinetic Response of Fishes to Different Levels of ...

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The Optokinetic Response of Fishes to Different Levels of Turbidity

By: Jeff Robbins

Major: Forestry, Fisheries, and Wildlife; Fisheries and Aquatic Sciences

Advisor: Dr. Suzanne Gray

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Abstract:

Many fish need light to school, reproduce, and forage. Without enough light penetrating into an

aquatic system fish may not be able to accomplish these tasks, thus compromising their ability to

persist. Turbidity, or suspended particles in the water, is a serious global problem due to

increased run-off from urban and agricultural activities. High turbidity has the potential to reduce

light to a point where fish are unable to detect the visual environment. The first objective to this

project was to develop an optomotor response apparatus for testing the visual abilities of fish

under increasing turbidity. Due to the optokinetic response, fish will swim with a rotating black

and white grating until the turbidity reaches a peak where they can no longer sense the white

striations, at which point the fish can no longer see the gradient and stops swimming (i.e.

detection threshold). Under the parameters of 8 rotations per minute, striations 35 mm wide, and

broad-spectrum lighting 15 individual fish showed responses to the rotating gradients. For the

second objective, I used the optomotor apparatus to test the detection threshold under increasing

turbidity for Pseudocrenilbrus multicolor victoria, a widespread East African cichlid fish that

experiences extremes of human-induced turbidity. All fish that exhibited the optokinetic

response (n = 15) were tested for a turbidity threshold. Males showed a significantly higher

turbidity threshold (mean ± s.e. = 61.94 ± 3.03) level compared to females (mean ± s.e. = 52.64

± 2.92). This research is beneficial because it can be applied to many different fish species

experiencing increases in turbidity above natural levels and may contribute to our understanding

of the mechanisms of population declines associated with increased turbidity.

Introduction:

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Throughout the world, aquatic biodiversity is being lost in large numbers due to many

anthropogenic effects and the impact on humans is starting to be apparent. In Lake Victoria, East

Africa,human-introduced species, such as the Nile Perch (Lates niloticus), have decimated

endemic species of fish (Ogutu-Ohwayo 1990). Overfishing of inland waters has also reduced

biodiversity in freshwater systems and humans are therefore suffering because of the lack of

resources available (Allan et al. 2005). One major cause of biodiversity loss in freshwaters is due

to high levels of human-induced turbidity in natural water systems (Dudgeon et al. 2005).

Turbidity is the cloudiness or haziness of water caused by suspended particles (Utne-Palm,

2002). This reduces water clarity and can be detrimental to water quality. For example, increased

turbidity can lead to decreased primary production because of decreased light penetration, which

in certain food webs can cascade up through trophic levels causing population problems for prey

species as well (Henley et al. 2005). Increased turbidity is a global problem resulting from

agricultural and urban runoff (Dudgeon et al. 2005). Increased turbidity occurs both naturally

and by human activities. When arid areas or landscapes with altered land use from deforestation

are impacted with heavy rains, flash floods can occur and large sediment loads can be displaced

and carried through waterways, drastically increasing turbidity. Increased turbidity can also be

caused by agricultural and urban runoff in the forms of algal turbidity and large sediment

displacements (Fichez et al. 1992). Regardless of the source, the impact of increased turbidity on

biodiversity is being detected frequently and is therefore in need of a better management

strategy.

Increased turbidity that is higher than natural levels has been shown to be a key factor in

the loss of freshwater biodiversity (Dudgeon et al. 2005). In impacted systems, elevated turbidity

can reach such high levels that it may restrict a fish’s ability to visually sense the surrounding

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environment (Kroger 2003). Freshwater habitats with different underwater light environments

influence how fish detect each other, dependent on which wavelengths of light are most

abundant (Fuller 2002). Suspended particles in the water scatter light and absorb different

wavelengths making certain, thus changing the spectrum of light (i.e. color of light underwater).

Many fish need light in order to forage, avoid predators, school, and reproduce; without light

many fish may therefore be limited to only a certain number of visually-mediated actions (Hogan

& Laskowski 2013). When fish are restricted to limited amounts of light they may experience

population declines due to the lack of functions being able to be performed (Pita et al. 2015). For

example, in Lake Victoria there were recently an estimated 500 unique species of cichlid fish

(Seehausen et al. 1997). Within the past decade Lake Victoria has rapidly eutrophied and water

clarity has reduced greatly. Many cichlid fish species use certain color cues to locate a

conspecific mate and to avoid mating with heterospecific mates. With the increased turbidity in

Lake Victoria, cichlids were less able to identify color species-specific color cues leading to

homogenization of the species flock. The lack of broad-spectrum lighting due to high turbidity

disrupted reproductive isolation between cichlid species, leading to decreased biodiversity

(Seehausen et al. 1997). With the problem of excessive turbidity increasing globally it is critical

to know the visual detection thresholds of fish experiencing altered underwater light

environments in order to conserve fish diversity. In very turbid waters fish can become limited in

their visual sensitivity due to the lack of clarity in the water (Mueller et al. 2010). We therefore

expect that for each individual or species there is a turbidity level at which the fish’s detection

threshold is reached, therefore hindering behaviorally mediated activities.

Research has shown that we can determine a fish’s detection threshold by taking

advantage of the optokinetic response (Maan et al. 2006). The optokinetic response is an innate

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physiological feature in which fish respond instinctively to a moving object (Sperry 1950). An

example of the optokinetic response would be t that an aquarium fish will follow the motion of

your finger around the glass as you move it in a figure eight fashion. Optomotor devices have

been used to detect the optokinetic response of fishes in a controlled setting. Maan et al. (2006)

used optomotor tests to assess the sensory drive hypothesis in cichlids: testing if different photic

environments contribute to the evolution of reproductively isolated species. The experiment

tested if the cichlids could detect light under alternating intensities and colors, representing

different turbid environments.

In this study we investigated two objectives. First, we wanted to evaluate if the

optokinetic response could be used to understand detection thresholds under turbid conditions.

Many studies have used the optokinetic response to test detection thresholds using different light

intensities or colors of light (Sperry 1950), however no one to our knowledge has directly

manipulated turbidity levels. In order to complete this task it was necessary to develop a

functioning optomotor apparatus to test the visual abilities of fish under increasing levels of

turbidity. The second objective of this study was to test the detection threshold under increasing

turbidity of Pseudocrenilabrus multicolor victoriae, a widespread East African cichlid that

experiences extremes of human-induced turbidity. The males and females of this sexually

dimorphic fish (Fig. 1 and 2) play very different roles in reproduction and so we expect that they

might have different detection thresholds.

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Methods:

Optomotor apparatus design

Multiple optomotor device designs have been used to detect the optokinetic response in fish and

each have similarities and differences (Maan et al. 2006). Maan et al. (2006) used different

wavelengths of light throughout different trials of the optokinetic response while in this

experiment all spectrums of light were used at all times. The design used in this experiment is a

unique set-up, but also has the major components of other optomotor apparatuses (Fig. 3). In this

design, there was a glass cylindrical tank that had a diameter of 20.32 centimeters and was

purposed to house the sample fish during trials. The tank was kept stable by the use of high

tensile strength clamps and para-cord draped over a utility rack. This design was strong enough

to hold the tank for hours while still being weighted with water. Underneath this suspended tank

was a standard record player. Above the tank we hung a broad-spectrum light that was used to

represent natural sunlight. Adjacent to the light was a mounted video camera (Canon Vixia HF

R600 High Definition Camcorder) to capture video of the optokinetic response of the fish being

tested. Placed around the tank and anchored to the record player was a black and white striped

gradient that acted as the trigger mechanism for the optokinetic response. The striations used for

this experiment were 35 mm wide and both black and white striations were the same width and

alternated one after the other (Maan et al. 2006).

Study Species

The study species used in this experiment was the cichlid species Pseudocrenilabrus multicolor

victoriae (Fig. 1 and 2). This species of cichlid is endemic to the Lake Victoria region of Eastern

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Africa and is found in different habitats throughout the region (Chapman et al. 2002). Some

habitats these fish are found in are stagnant and clear swamp systems. P. multicolor can also be

found in moving water systems that are very turbid (Chapman et al. 1996). The male fish of this

species are normally a vibrant yellow color with prominent blue lips and other blue and red

markings on the anal fins (Fig. 1). They females are gray-brown with tinges of blue on the scales

and fins (Fig. 2). Males use their vibrant colors to display for a potential mate, or they use them

to defend their territories against other males who could be potential threats (Gray et al. 2012). P.

multicolor uses vision as a primary sense in reproduction. By limiting vision due to turbidity this

species may suffer due to the fact that they may not be able to detect each other in turbid versus

clear water. The specific fish used for this experiment were either first generation P. multicolor

collected in Uganda or an F1 generation of a mixed population from parents originating from

swamp or river habitats.

Visual Detection Trials

Trials were executed after the design of the optomotor device was completed and we had

established that the fish would respond to a grating of 35 mm size and speed of 8 rotations per

minute (Maan et al. 2006). Different acclimation periods were conducted in the preliminary trials

to determine which time period guaranteed the optokinetic response (acclimation times = 10

minutes and 30 minutes). Smaller gradient sizes were also tried to determine which width of

gradients generated the most apparent response. Each fish was placed in 800 ml of water in the

cylindrical tank and let to sit for a 15-minute acclimation period. Once the fish was settled the

gradient was activated to spin around the tank at a speed of 8 rotations per minute (Maan et al.

2006). The fish demonstrated the optokinetic response for 2 minutes in clear water before any

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turbidity solution was added. The turbidity solution was created by mixing 5g of bentonite clay

with 50ml of treated water (NovAqua). The turbidity solution was added in increments of 0.08

mL, which increased turbidity in the tank approximately 2 NTUs for every addition of turbidity

solution (Fig. 4). The water within the tank was homogenized with the turbidity solution by

recirculating the water through a 30mL pipette after each addition. The turbidity solution was

incrementally added until the fish within the tank stopped swimming in the clockwise rotation

and responding to the black and white gradient. A total of 15 fish were tested for turbidity

thresholds and each fish was run through two trials to have a standardized result, and both of

those measurements were used to calculate a mean NTU threshold level for each fish.

Results:

The overall design of the optomotor apparatus was successful in generating the

optokinetic response from P. multicolor. With gradients 35 mm wide and spinning at a speed of

8 rotations per minute the sample fish responded in clear and turbid water. In the preliminary

trials 5 fish were used to generate an optokinetic response in only clear water to solidify the

capabilities of the device and the parameters being used.

The optokinetic response was generated in 15 different fish consisting of 8 females and 7

males (Table 1 and 2). The mean (± S.E.) detection threshold for all fish was 57.02 (± 2.4) NTU.

The average threshold level for males was 61.94 (± 3.0) NTU (Fig. 5). The average threshold

level for females was 52.64 (± 2.9) NTU (Fig. 5). A two-tailed unpaired t-test was conducted

between male and female turbidity threshold. The t-test was done with a 95% confidence interval

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and generated a t-value = 2.2025 and a P-value =0.046, indicating a statistically significant

difference between male and female turbidity thresholds.

Discussion:

A successful optomotor apparatus was designed through the creative and repurposed use

of materials. The novel idea of using a record player to rotate the striped gradient worked very

well. The optomotor device was able to successfully generate an optokinetic response in the

available fish specimens; P. multicolor had a physiological response to the optomotor apparatus.

The turbidity was successfully increased throughout trials to show how fish react in different

turbid settings. One notable observation from the experiment was that males showed higher

turbidity threshold levels than females (Fig. 3). One conclusion that can be made from this is that

males are therefore able to visually sense females before females detect males under turbid

conditions; potentially giving males the ability to court females with a colorful behavioral

display before other males compete for the same female mate. The levels of turbidity produced

simulated natural settings from human induced changes in the environment (Kasangaki et al.

2008).

The idea of using an optomotor device for testing visual acuity in turbid waters is just one

way to test responses to turbidity. Other designs have focused on the concept of reaction distance

and have used long rectangular tanks instead of cylindrical ones. The optomotor response has

been used in many different fish and mammals (Sperry 1950), but this experiment was the first

time that fish have been tested with the optokinetic response to analyze their response to

turbidity. In another experiment, Gregory & Northcote (1993) used reaction distance to

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determine when juvenile Chinook Salmon (Oncorhynchus tshawytscha) would react to a prey

source and observed that reaction distance decreased as turbidity increased. The advantage of

using the optokinetic response to test turbidity thresholds compared to reaction distance is that

turbidity can be changed during the actual trial using an optomotor apparatus. By being able to

increase turbidity during a trial you can increase NTUs in smaller increments and therefore get a

more accurate determination of the turbidity threshold. Turbidity scatters light and makes objects

harder to detect (Utne-Palm 2002). By using a broad-spectrum light in this experiment we were

able to simulate a natural, daylight setting and proved that with increased turbidity organisms

that rely on vision become significantly visually impaired at certain levels of turbidity. Using the

optomotor apparatus will allow other species of fish to be analyzed in a controlled environment

and determine their turbidity thresholds. High levels of human induced turbidity are certainly a

global issue. Different fish species may be encountering population dynamic problems and the

issue could be excessive turbidity (Henley et al. 2000). By testing different species of fish using

the optokinetic response to turbidity, management plans can be made and enforced to

compensate for the overwhelming turbidity in areas that fish are facing severe problems.

Acknowledgements

Thank you to The Ohio State University and The School of Environment and Natural Resources.

Thank you to Dr. Suzanne Gray and my peers in her Lab. IACUC Protocol Number:

2014A00000055

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Kroger, R. H. H. 2003. Rearing in different photic and spectral environments

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Fig2.FemaleP.mulitcolor

Fig1.MaleP.multicolor

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Figure3.ModifieddesignoftheoptomotorapparatusfromMaanetal(2006).Thecylindricaltankissuspendedaboveastandardrecordplayerusingpara-cordtiedtoclampsthatareattachedtotheedgeofthetank.Thisdesignholdsthetanklevelabovetherecordplayer.Ontopoftherecordplayersitsthegradientthatspinsaroundthesuspendedtank.Hangingdirectlyabovetherecordplayerisabroad-spectrumlight.Adjacentthelightisavideocameratocapturethebehaviorofthefish.

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Figure4.StandardizedadditionsofturbiditysolutionandtheirchangeoftheNTUtotheexistingwater.Abentoniteclaysolutionwasmadetoincreaseturbidity.Theclaysolutionwasaddedwithastandard2.5mLpipette.Everyadditionofthesolutionwas0.08mLandincreasedtheturbidityapproximately2NTUs.

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6 0.8 1 1.2 1.4

NTU

mLofTurbiditySolution

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Female Fish ID NTU Threshold

2 52.78

3 44.73

4 44.17

5 53.86

7 48.95

9 59.59

10 68.84

14 48.24

Table1.Femalefishspecimensandtheiraveragevisualthresholds

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Male Fish ID NTU Threshold

1 60.81

6 69.17

8 65.53

11 48.17

12 69.55

13 65.94

15 54.45

Table2.Malefishspecimensandtheiraveragevisualthresholds

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0

10

20

30

40

50

60

70

Female Male

TurbidityThreshold(NTU)

Figure5.Themean(±s.e.)visualacuitythresholdsformaleandfemaleP.multicolorunderincrementallyincreasedturbidity(NTU)